1. Field of the Invention
The present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs), and more particularly relates to starting a fluorescent lamp with improved efficiency.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate the light is limited. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFLs are used for back lighting or edge lighting of liquid crystal displays (LCDs), which are typically used in notebook computers, web browsers, automotive and industrial instrumentations, and entertainment systems. Such fluorescent lamps require a high starting voltage (on the order of 700-1,600 volts) for a short period of time to ionize the gas contained within the lamp tubes for ignition. After the gas in the CCFL is ionized and the CCFL is fired, less voltage is needed to keep the CCFL on.
A CCFL tube typically contains a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube, which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphorescent material coated in the inner wall of the tube, resulting in visible light.
A power conversion circuit is generally used for driving the CCFL. The power conversion circuit accepts a direct current (DC) input voltage and provides an alternating current (AC) output voltage to the CCFL. The brightness (or the light intensity) of the CCFL is controlled by controlling the current (i.e., the lamp current) through the CCFL. For example, the lamp current can be amplitude modulated or pulse width modulated to control the brightness of the CCFL.
One type of power conversion circuits includes a resonant circuit. The power conversion circuit includes switching transistors in a half bridge topology or a full bridge topology using power metal-oxide-semiconductor-field-effect-transistors (MOSFETs) to provide the DC to AC conversion. Maximum power is provided at the output of the power conversion circuit by switching the MOSFETs with driving signals at a resonant frequency. To control the output voltage as well as the current through the lamp, the power conversion circuit can change the frequency of the driving signals either towards the resonant frequency or away from the resonant frequency.